The present invention relates to a system for automatically controlling a vehicle braking system to inhibit wheel locking, i.e., to a so-called anti-skid control system. The invention is particularly, but not solely, concerned with motor vehicles especially commercial vehicles with pneumatic braking systems. Various forms of anti-skid systems have been proposed which operate electrically. Generally such systems are expensive and a general object of this invention is to provide a non-electrical system which is relatively cheap and simple yet reliable and robust in operation.